1. Field of the Invention
The present invention pertains to scanning optical systems for reading information recorded in a magnetic recording medium, and more particularly, it pertains to such systems which utilize a polarized light source and a detector for detecting the slight rotation of light caused by the Faraday or polar Kerr effect when the light is reflected from a previously magnetized area on the medium.
2. Description of the Prior Art
Erasable magneto-optic recording is viewed as a possible successor to electromagnetic recording that is used today in high performance disk drives for main frame computers, hard disks, and floppy disks for Personal computers and magnetic tape drives. Magneto-optic recording offers bit densities (approximately 10.sup.8 bits per square centimeter) ten times that of high performance disk drives and fifty to one hundred times the density of the low-end disk drives. The optical recording and reading head is typically positioned at least one millimeter above the disk surface which arrangement eliminates the possibility of head crashes and also allows the disk to be readily removed. Rigid plastic disks with grooved recording tracks on 1.6 micron centers can be rotated at speeds as high as conventional Winchester drives. Since bit density along the track is comparable to today's Winchester technology, data rates can be the same.
Thermomagnetic recording is used in magneto-optic recording technology for writing information on the storage medium. The medium material is heated above a critical temperature by a diode laser beam the energy of which is concentrated on a very small spot on the target at the focal point of the beam, and the heated spot is allowed to cool in a magnetic field. The direction of the magnetic field, which is normal to the disk surface, determines the direction of the magnetization in the recording. In writing on the disk a diode laser provides the heat source by means of a beam which is passed to an objective lens to focus the beam on the spot on the disk. A bias magnetic field is provided by a coil positioned adjacent to the disk and operable to selectively vary the direction of magnetization as the disk is rotated with respect to the beam.
In reading the information recorded upon the disk, the directions of magnetization are detected by a focused laser beam utilizing either the polar Kerr or the Faraday magneto-optic effect. When a linearly polarized light beam is reflected from the surface of a vertically magnetized material there is a rotation of the plane of polarization (the polar Kerr effect). The sense of the rotation depends upon the direction of magnetization in the magnetic material. In a typical optical reading system, a laser beam is first linearly polarized and then passed through a beam splitter and an objective lens which focuses the polarized beam on the surface of the magneto-optic disk. The beam reflected from the magnetic material surface will have its plane of polarization rotated about 1/4 to 1/2 degree due to the polar Kerr magneto-optic effect with the direction of rotation being due to the direction of magnetization of the disk surface, e.g., with a positive polarization plane rotation (angle X) indicating a binary "one" and a negative rotation (angle -X) indicating a binary "zero". The polarization rotated reflected beam passes back through the objective lens to the beam splitter wherein a portion of it is deflected to an analyzer which has its polarization direction set perpendicularly to the polarization direction of one of the polarization rotated beams reflected from the disk, e.g., set perpendicularly to the angle -X. Thus the analyzer discriminates between the two polarization rotated beams by producing essentially no output beam for the -X beam and producing an output beam equal to the sin 2X component of the +X beam. A conventional photodetector reads the analyzer output to provide the indicated digital output signal, i.e., a "one" or a "zero" for respectively positive or negative polarization angle rotation. Alternatively, the information (i.e., "ones" or "zeros") can be provided by areas of magnetization and non-magnetization on the magneto-optic disk surface. Analysis can then be made by simply discriminating between a plane polarization rotated reflected signal and a non-rotated reflected signal (e.g., by a single polarizer).
Examples of typical optical systems used in magneto-optic reading systems of the prior art are shown in U.S. Pat. No. 4,409,631 to Matsumoto.